Optical switches are well known in the electronics industry. These devices employ an optical emitter which provides a light source and an optical receiver or sensor which produces an electrical output in response to receiving the emitted light. The emitter is usually a LED (light emitting diode) and the receiver is usually a photodiode electrically connected to the base of a transistor, thus forming a light sensitive transistor which operates as an optical switch. Optical switches consisting of matched optical pairs (one emitter and one receiver) enclosed within a molded case are commercially available from a number of electronic component manufacturers such as Sharp Electronics Corporation and Optek Technology Inc. These optical switches are constructed such that a generally semispherical lens of the emitter is directly opposite a generally semispherical lens of the receiver thereby permitting light emitted from the emitter to be easily received by the receiver. The emitter emits a quantity of light energy relative in strength to an electric current passing through the emitter. The photodiode of the receiver transforms light energy into an electrical output which is proportional to the amount of light energy received. This electrical output is applied to the base of the associated transistor. When the electrical output is equal to or greater than the base threshold level of the transistor, it begins to conduct. This causes a detection circuit connected to the transistor to indicate an ON state to any device being controlled by the optical switch. The two lenses are spaced apart such that an operator made from an opaque material can be slidably positioned between them thereby preventing light from the emitter from being received by the receiver. When the light received by the receiver is not sufficient to produce an electrical output equal to or greater than the base threshold level of the transistor it will not conduct, and the detection circuit will indicate an OFF state to any device controlled by the optical switch. The receiver is switched between these two states by selectively moving the operator such that it is either between or not between the two lenses of the optical pair. In many applications, the precise linear displacement of the operator required to produce a guaranteed OFF-point (point at which no light from the emitter is received by the receiver) is required. This linear displacement is generally measured with respect to a reference surface on the device enclosure. Since ambient light received by the receiver can affect the OFF-point, and since the lenses of the emitter and receiver are semispherical in shape, thereby emitting and receiving light from all directions, the linear displacement of the operator at the guaranteed OFF-point can vary greatly from one device to another. This is true even for devices of the same design and manufacturer's catalog number.
When precise linear displacement of the operator at the guaranteed OFF-point with respect to the reference surface is critical, it has been common practice to use more expensive apertured optical switches. The apertures are very narrow slits placed directly in front of the lenses of one or both of the emitter and receiver, thereby limiting the direction and amount of light that can be received. The aperture placed in front of the emitter lens is generally wider than the aperture placed in front of the receiver lens. Matched optical pairs are either overmolded with an opaque material leaving only the narrow apertures in front the lenses or are placed in an auxiliary housing having apertures integrally formed in the housing walls. Apertured optical switches do have a more precisely controllable OFF point and a more stable OFF state than unapertured optical switches. However, the apertures cause other problems which can effectively defeat their intended purpose. The overmolded apertures or auxiliary apertured housings add additional mechanical tolerances to the linear operating mechanism of the optical switch. These additional mechanical tolerances reduce the chance that the linear displacement of the operator at the guaranteed OFF point, with respect to the reference surface, will be consistent between individual optical switches. Apertures also reduce the amount of light which can be received by the receiver during the full ON state which in turn reduces the base current of the receiver transistor. Since the gain of the receiver transistor is proportional to the amount of light received, an apertured optical switch will have lower gain than an unapertured optical switch, resulting in a narrower range or difference between the guaranteed minimum ON and minimum OFF signals from the receiver. In both apertured and unapertured optical switches, the dark current (leakage current in the transistor when no light is detected) can vary significantly from one optical switch to another of the same design and manufacturer's catalog number. The dark current causes a DC offset in the output of the optical switch. The DC offset, in combination with the narrower range between guaranteed minimum ON and OFF signals caused by lower gain of an apertured optical switch, can cause a fixed threshold DC detection circuit to fail to detect an ON or OFF state of the optical switch. In a worst case scenario, the dark current of one optical switch can be greater that the guaranteed minimum ON state of another optical switch. In this situation the ON state would not reach the threshold of the DC detection circuit and therefore the DC detection circuit would not indicate a change in state of the optical switch. One solution to this problem is to use an AC detection method which is not affected by the variances of the DC offset. The AC detection method is more expensive than a DC detection method and again increases the cost of producing an optical switches with a precisely repeatable guaranteed OFF point and stable OFF state. A more desirable solution would be to provide the less expensive higher gained unapertured optical switch with a means for obtaining a guaranteed OFF-point at a precise linear displacement of the operator which can be consistently repeatable from one device to a another within a family of devices or a common manufacturer's catalog number. It would also be desirable to provide a means that would ensure sufficient gain in the receiver transistor such that a wider range or difference between the guaranteed ON and OFF signals is produced, thus allowing the less expensive DC detection method to be used.